In order to improve reliability and enhance the current supply capability in a filter having a common-mode choke coil, use is made of an arrangement (a redundant arrangement) in which a plurality of filter circuits alone or modules in which a filter circuit is additionally provided with a voltage monitoring function or over-current protection function (these are referred to as “filter modules”) are disposed in parallel.
FIG. 4 is a diagram illustrating a typical example of an arrangement in which a plurality of filter circuits are connected in parallel. It should be noted that an X capacitor (across-the-the capacitor), Y capacitor (line-bypass capacitor) and normal-mode choke coil, etc., are not shown in FIG. 4.
As shown in FIG. 4, a filter comprising a common-mode choke coil (abbreviated to “CMC”) 1a and resistors (7a, 8a) and a filter comprising a common-mode choke coil (abbreviated to “CMC”) 1b and resistors (7b, 8b) are arranged in parallel between an input and an output. The CMC functions as a filter that removes only common-mode noise without affecting a differential signal. More specifically, when a differential current flows, magnetic fluxes produced respectively by the two coils cancel each other out within the cores. When a common-mode current flows, the fluxes of the two coils strengthen each other within the cores, impedance across the ends of the coils is enlarged and the common-mode current is suppressed. Suppression of noise is the result.
By way of example, with regard to current on the positive side, the positive-side winding current of CMC la and the positive-side winding current of CMC 1b are not equal owing to a slight difference between the circuit resistance values 7a and 7b. Similarly, with regard also to the negative-side windings, the negative-side winding current of CMC 1a and the negative-side winding current of CMC 1b are not equal owing to a slight difference between the circuit resistance values 8a and 8b. 
In a case where the ratios between the differences of the circuit resistance values (7a:7b, 8a:8b) are equal, the currents that flow into the CMC 1a and CMC 1b are different but the positive-side winding current and negative-side winding current when viewed in terms of each individual CMC are equal.
In actuality, however, this state is almost never attained and it is difficult to equalize the current that flows into the positive-side winding and the current that flows into the negative-side winding of each individual CMC used in the filter.
If the currents that flow into the positive-side and negative-side windings are not equal (i.e., if a current imbalance occurs), the core material used in the CMC saturates magnetically, inductance drops by a significant margin and a normal filter characteristic is no longer obtained.
As a technique relating to adjustment of current imbalance, Patent Document 1 discloses an arrangement in which ripple in a parallel-operating-type switching power supply unit is suppressed by equalizing reactor currents that flow into respective output reactors. The arrangement described in Patent Document 1 is entirely different from the filter circuit of the present invention, described later.
[Patent Document 1] Japanese Patent Kokai Publication No. JP-P2003-274651A
The entire disclosure of Patent Document 1 is incorporated herein by reference thereto. The following analyses are given by the present invention.
In the arrangement in which a plurality of filter circuits using CMCs are connected in parallel, as mentioned above, there are cases where operation stabilizes at different values for the positive-side and negative-side currents in each filter module. In such cases the inductance of the CMC within the filter module declines and the desired filter characteristic (low-pass characteristic) is not obtained.
More specifically, when a difference is produced between the positive and negative winding currents of the filter and an imbalance develops between the positive and negative winding currents of the CMC, a difference develops between a magnetic flux generated by the positive winding current and a magnetic flux negated by the negative winding current, which fluxes had been canceled out within the two coils of the CMC, magnetic fluxes are produced within the CMC core, the CMC core reaches magnetic saturation (the higher the magnetic permeability, the greater the tendency to saturate). Inductance declines owing to the saturation of the CMC core, high-frequency impedance of the CMC decreases and so does the amount of attenuation of common-mode noise of the filter. The cut-off frequency of common-mode noise rises. As a result, common-mode noise output from the apparatus increases and there is a decline in immunity to common-mode noise entered into the apparatus from the outside.